Freezing nucleant

ABSTRACT

A solid solution of silver iodide and cuprous iodide for the purpose of more effectively nucleating the formation of ice at higher temperatures than the nuclei in the environment is provided. These compounds may be dispersed into the atmosphere with similar equipment to that used for the dispersal of silver iodide, and the resultant aerosol is equal to or superior to silver iodide for the purpose of nucleating clouds while being less toxic, less expensive and less susceptible to photodeactivation by sunlight than pure silver iodide.

United States Patent 11 1 Vonnegut et a1.

[4 1 Apr. 15, 1975 FREEZING NUCLEANT [75] Inventors: Bernard Vonnegut,Albany, N.Y.;

Henry Chessin, Chicago, 111.; Richard E. Passarelli, Jr., Schenectady,NY.

[73] Assignee: The United States of America as represented by theSecretary of the Navy, Washington, DC.

22 Filed: Aug. 9, 1974 211 Appl. No.: 496,438

[52] U.S. Cl 239/2 R [51] Int. Cl AOlg /00 [58] Field of Search 239/2 R,14

[56] References Cited UNITED STATES PATENTS 3,126,155 3/1964 Lohse239/14 3,l27,l07 3/1964 Merryweather 239/2 R 3,545,677 12/1970 Power239/2 R 3,788,543 1/1974 Amand et al. 239/14 Primary ExaminerLloyd L.King Attorney, Agent, or FirmR. S. Sciascia; C. E. Vautrain, Jr.

[5 7] ABSTRACT 8 Claims, 3 Drawing Figures '3. 0 o 0 ICE g 4.4- o o E, ll l l l l J 0 I0 3o so so so I00 MOLE PERCENT CUI lN AgI FREEZINGNUCLEANT This invention concerns nucleation catalysts and. moreparticularly, an improved control means which reduces the lattice misfitbetween ice and silver iodide.

One of the several explanations offered for the effectiveness of silveriodide in initiating the freezing of supercooled water is that itscrystal structure is similar to that of ice. Another explanation is thatsilver iodide. as usually prepared. has 0.07 to 0.37 percent of itssurface area covered with hydrophilic sites and the balance covered withhydrophobic sites.

It has also been discovered that when burned on the surface of aninsoluble oxide such as silica. sodium or potassium halides and iodatesare unusually effective in modifying insoluble substrates which would bewholly hydrophilic or wholly hydrophobic. to create a relation of areasand spacing of hydrophilic and hydrophobic sites which are favorable tofreezing nucleation by providing a ratio of monolayer volume byisopropanol divided by a monolayer volume for water between 0.35 and0.75, preferably between 0.4 and 0.6 and most desirably about 0.5. Anyone ofa wide variety of solid insoluble oxides may be employed such asrelatively dense solid precipitated silica and other solid oxidesinsoluble in water such as magnesia. alumina, hematite and titania.

It has further been found that within the same temperature interval assilver iodide and lead iodide the following substances in dispersed formact as iceforming nuclei copper iodide (Cul). cupro-oxide (Cu O). coppersulphide (CuS), copper selemide (CuSe). mercury telluride (HgTe),vanadium pentoxide (V silver sulphide (AgS). silver nitrate (Ag- NOsilver oxide (Ag O) and cadmium telluride (CdTe). Of these compounds,only copper sulphide and cupro oxide were found practical forinfluencing weather because of the toxicity or excessive cost of theremaining compounds. among other deficiencies. Applicants have divergedfrom the direction taken in prior searches for more efficaciousnucleants and have discovered a less toxic, less expensive and moreeffective seeding material by achieving a better crystal match for thelattice misfit between ice and silver iodide.

Accordingly, it is an object of the present invention to provide animproved nucleation catalyst for cloud seeding.

Another object of the invention is to provide for improved cloud seedingby reducing the lattice misfit between ice and silver iodide.

A further object of the invention is to provide for improved cloudseeding by an improved crystal match in a composition which includessilver iodide and reduces some of the undesirable aspects of thatsubstance.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description thereof whenconsidered in conjunction with the accompanying drawings in which likenumerals represent like parts throughout and wherein:

FIG. 1 is a graph showing changes in lattice spacing due to changes inthe mole percent of Cul and Ag]:

FIG. 2 is a graph showing the observed threshold nu cleation temperatureas a function of composition of both fusions and precipitates; and

FIG. 3 is a graph illustrating the symmetry of data of catalysts ofvarying disregistry.

It will be noted that although cuprous iodide (Cul) has been listed inthe prior art as one of several substances which in dispersed form actas ice-forming nuclei. this and many other listed substances have beendiscounted as impractical or unsuitable because of their poisonouscharacter or for economic reasons.

In contrast to the findings in the prior art we have found that althoughby itself cuprous iodide has undesirable features such as having beendetermined to be somewhat toxic. the combination of cuprous iodide andsilver iodide in a solid solution is appreciably less toxic than silveriodide alone. This is believed due to the fact that copper is less toxicthan silver. Additionally, the Cul-Agl compound is less expensive thansilver iodide and, therefore, the cost of seeding with the compoundwould be appreciably less than that of seeding with pure silver iodide.It is also believed that the Cul-Agl compound is less susceptible tophotodeactivation than pure silver iodide and. therefore, an increase inthe efficiency of a seeding operation should be achieved due to a longerresidence time in the atmosphere.

In accordance with our invention, we have found that as the proportionof cuprous iodide in a Cul-Agl nuclei is increased to approximately 25mole percent. the supercooling required to initiate ice formationdecreases from 2C for pure silver iodide to a minimum value ofapproximately O.5C. The cuprous iodide is formed in crystals of a solidsolution of silver iodide and cuprous iodide and nucleates the formationof ice in supercooled water. Upon further increase of the percentage ofCul. the supercooling again increases until it reaches a value on theorder of 25C for pure Cul. indicating a superiority of the Cul-Aglcrystals over the Agl crystals alone used commonly for seedingsupercooled clouds. Compounds of the solid solution of silver iodide andcuprous iodide can be dispersed into the atmosphere with similarequipment to that used for the dispersion of silver iodide, and theresultant aerosol is equal to or superior to silver iodide for thepurpose of nucleating clouds.

Cul-Agl solid solutions have been prepared each having a latticeparameter smaller than that of pure Agl using two separate methods. Thefirst method was direct fusion at a pressure of 10 torr and atemperature of 20C above the melting point of the solid solution, about600C. The second involved dissolving the proper proportions of reagentgrade Cul and Agl in hydriodic acid and then boiling off the solvent.The crystal class of the resulting solid solutions is face centeredcubic, but in the (111) plane the crystal structure is similar to thatof ice in the (0001) plane. The lattice spacing in the (1 l 1) plane isgiven by a V272, where a is the edge of a unit cell in the cubic system.The graph in FIG. 1 illustrates that by varying the mole percent of Culin Agl. a solid solution can be made to have any lattice spacing from4.58 to 4.28 A measured in the l 1 l plane. This range in latticespacings corresponds to lattice disregistries with respect to iceranging from 1.5 percent to -5.2 percent. Since Cul and Agl are notappreciably soluble in water, 10 and l0 mole/liter, respectively, theyhave a negligible effect in depressing the freezing point of the water.

To determine their relative effectiveness as nucleation catalysts.powdered samples of about 0.5 of Cul- AgI were sealed in dilatometerscontaining approximately 1 g of distilled water, and the freezingtemperature was observed when they were cooled in a vigorously stirredbath. The temperature was measured with a thermometer with an accuracyof 0.0lC and was lowered at a rate of 0.5C min. The water was slightlydistilled and had a conductivity of 3 X IO ohm cm.

The graph of FIG. 2 shows the observed threshold nucleation temperatureas a function of composition for samples of both the fusions and theprecipitates. The fusions were tested immediately' after having beenimmersed in water. Reduction of the cooling rate from 0.05C to 0.0lC minproduced no observable differences in threshold nucleation temperatures.

FIG. 3 presents curves for the smallest observed supercooling for theCul-Agl fusions and precipitates at 31 and 32, respectively. as afunction of lattice disregistry with respect to ice. The disregistry iscomputed for the (I ll) plane of the CuI-Agl solid solution and the(000i plane of ice. A curve for precipitated Agl-AgBr solid solutions isshown at 33 along with a theoretical curve for coherent nucleation at34.

It is believed that the interfacial energy between ice and a nucleationcatalyst is a minimum when nucleation is coherent. Coherent nucleationoccurs when the lattice of the forming crystal matches the lattice ofthe catalyst and can be likened to crystal growth. If the lattices areslightly mismatched, nucleation may still be coherent. but the resultingelastic distortion will increase the bulk free energy of the formingembryo. Consequently, the supercooling necessary for nucleationincreases by an amount proportional to the square of the disregistry.

The symmetry of the data in FIG. 3 may indicate that catalysts with apositive disregistry have about the same nucleation ability as thosewith an equal negative disregistry. This result agrees with a predictionthat AT k5? where AT is the supercooling, 8 is the disregistry, and k isa constant. However. for our data the supercooling does not approach 0Cas the disregistry tends to zero, but instead reaches a minimum of aboutO.5C. When other nucleation parameters are taken into account. theequation for coherent nucleation can be written as AT= k6 AT where AT isthe supercooling due to the combined effect of any nucleation parametersindependent of the disregistry. The parallel nature of the data in FIG.3 tends to confirm this type of relation.

Coherent nucleation theory predicts that nucleation will becomeincoherent for large disregistries where the bulk free energy of iceformation is less than the free energy required to distort the icelattice to fit the catalyst lattice. If nucleation is not coherent, alinear relation can be predicted between the disregistry and thesupercooling. The data for the Cul-Agl catalysts in FIG. 3 show a sharpdip in supercooling between lattice disregistries of $1.5 percent. Thisregion could correspond to coherent nucleation. The region between l .5and 5.2 percent disregistry could correspond to incoherent nucleation,as the supercooling is a fairly linear function of the disregistry.

Since the operation and use of cloud seeding nuclei is based on theability of cuprous iodide and silver idoide to nucleate ice formation athigher temperatures than the nuclei normally present naturally, thenewly developed seeding material consisting of copper iodide and silveriodide makes possible the use of nuclei that are significantly moreeffective than those which have been available in the past. Furthermore,since in some cases it may be desirable to control the nucleationtemperature to any desired value, the continuous spectrum of nucleiobtainable by varying the composition of the solid solution provides anew tool for use in cloud seeding technology.

What is claimed is:

1. A method of nucleating ice formation by cloud seeding nuclei athigher temperatures than those for the nuclei normally present naturallycomprising:

reducing the lattice misfit between ice and silver iodide bysubstituting other metallic atoms for a portion of the silver atoms inthe silver iodide lattice.

2. The method of claim 1 wherein the amount of lattice misfit is from 1.5 percent to +1.5 percent with respect to ice.

3. The method of claim 2 wherein the other metallic atoms are copperatoms and the proportion of cuprous iodide in a Cul-Agl nuclei issubstantially 25 mole percent.

4. The method of inducing crystal formation in a natural atmosphericcloud which is supercooled naturally to between 4C and 5C whichcomprises:

introducing into said cloud an aerosol comprising crystals of silver andanother metallic iodide sufficient to further supercool said cloudsubstantially O.5C thereby initiating ice formation therein. 5. Themethod defined in claim 4 wherein the other metallic iodide is cuprousiodide and the proportion of cuprous iodide in the Cul-Agl nuclei ofsaid crystals is substantially 25 mole percent.

6. The method of inducing crystal formation in a mass having atemperature of from 4C to 5C and below which comprises:

preparing a solid solution having a lattice parameter smaller than thatof pure silver iodide by the direct fusion of another metallic iodideand silver iodide at a pressure of substantially 10* torr and atemperature substantially 20C above the melting point of said solution;and

adjusting the mole percent of the other iodide in the silver iodide soas to produce a lattice spacing in the l l l plane of from substantially4.58 A to substantially 4.28 A, whereby lattice spacings are producedwhich correspond to lattice disregistries with respect to ice rangingfrom L5 percent to 5.2 percent.

7. The method defined in claim 6 wherein the other iodide is cuprousiodide and the proportion of cuprous iodide in the solution issubstantially 25 mole percent.

8. The method of inducing crystal formation in a mass having atemperature of from 4C to 5C and below which comprises:

preparing a solid solution having a lattice parameter smaller than thatof silver iodide by dissolving selected proportions of cuprous iodideand silver iodide in hydriodic acid and thereafter boiling off thesolvent, said proportions determining the mole percent of cuprous iodidein silver iodide so as to produce a lattice in the (l 1 1) plane of fromsubstantially 4.58 A to substantially 4.28 A,

whereby lattice spacings are produced which correspond to latticedisregistries with respect to ice ranging from 1.2 percent to 5.2percent.

1. A METHOD OF NUCLEATING ICE FORMATION BY CLOUD SEEDING NUCLEI ATHIGHER TEMPERATURES THAN THOSE FOR THE NUCLEI NORMALLY PRESENT NATURALLYCOMPRISING: REDUCING THE LATTICE MISFIT BETWEEN ICE AND SILVER IODIDE BYSUBSTITUTING OTHER METALLIC ATOMS FOR A PORTION OF THE SILVER ATOMS INTHE IODIDE LATTICE.
 2. The method of claim 1 wherein the amount oflattice misfit is from -1.5 percent to +1.5 percent with respect to ice.3. The method of claim 2 wherein the other metallic atoms are copperatoms and the proportion of cuprous iodide in a CuI-AgI nuclei issubstantially 25 mole percent.
 4. The method of inducing crystalformation in a natural atmospheric cloud which is supercooled naturallyto between -4*C and -5*C which comprises: introducing into said cloud anaerosol comprising crystals of silver and another metallic iodidesufficient to further supercool said cloud substantially 0.5*C therebyinitiating ice formation therein.
 5. The method defined in claim 4wherein the other metallic iodide is cuprous iodide and the proportionof cuprous iodide in the CuI-AgI nuclei of said crystals issubstantially 25 mole percent.
 6. The method of inducing crystalformation in a mass having a temperature of from -4*C to -5*C and belowwhich comprises: preparing a solid solution having a lattice parametersmaller than that of pure silver iodide by the direct fusion of anothermetallic iodide and silver iodide at a pressure of substantially 10 2torr and a temperature substantially 20*C above the melting point ofsaid solution; and adjusting the mole percent of the other iodide in thesilver iodide so as to produce a lattice spacing in the (111) plane offrom substantially 4.58 A to substantially 4.28 A, whereby latticespacings are produced which correspond to lattice disregistries withrespect to ice ranging from 1.5 percent to -5.2 percent.
 7. The methoddefined in claim 6 wherein the other iodide is cuprous iodide and theproportion of cuprous iodide in the solution is substantially 25 molepercent.
 8. The method of inducing crystal formation in a mass having atemperature of from -4*C to -5*C and below which comprises: preparing asolid solution having a lattice parameter smaller than that of silveriodide by dissolving selected proportions of cuprous iodide and silveriodide in hydriodic acid and thereafter boiling off the solvent, saidproportions determining the mole percent of cuprous iodide in silveriodide so as to produce a lattice in the (111) plane of fromsubstantially 4.58 A to substantially 4.28 A, whereby lattice spacingsare produced which correspond to lattice disregistries with respect toice ranging from 1.5 percent to -5.2 percent.